[wip] Basic PoC of Expand type for TypedDict-kwargs

[untitaker]

This implements Expand in kwarg positions as per #4441. TODO:

• Forbid usage of Expand outside of *args and **kwargs (what does x: Expand[...] = 42 even mean?)
• Implement Expand properly s.t one can import it at runtime
• Commit the changes to mypy_extensions
• tests

Right now this PR only exists as a discussion basis for the approach I should take when implementing this properly. Adding expand_formal_type seems to be the best way to me right now but I don't have good oversight over the entire codebase to know any better.

[untitaker]

This is ugly. How do I get something like TypedDictType (ExpandType) and make mypy map mypy_extensions.Expand to mypy.types.ExpandType?

[ilevkivskyi]

Thanks for PR!

I think it should be done in a different way: it will be simpler if most of things happen during semantic analysis rather than type checking. Note that we currently have two semantic analyzers (things that construct symbol tables and bind reference expressions to symbols). The old one will be deprecated soon, so I propose to only support this with the new one (located in mypy/newsemanal). Here are some comments:

• Expand should be prohibited everywhere except in **kwargs.
• This should not be a proper type but rather a name qualifier (like Final or ClassVar)
• Type analyzer should modify function in place during semantic analysis using the expanded version (see mypy.newsemanal.typeanal.TypeAnalyzer.visit_callable_type)
• At the same time, the analyzer should insert a kwargs variable (having the actual explicit TypedDict type) in the local symbol table, see around here.

Let me know if you have more questions.

[untitaker]

I think I don't have the motivation to work on this anymore, sorry.